Radio equipment (e.g., keyless receivers) used in, for example, vehicles and dwellings uses radio waves in ranges, such as UHF and VHF bands, of relatively long wavelengths (several tens of centimeters to several meters). In the construction of such radio equipment, the physical size of the radio equipment is governed by the size of an antenna. To reduce the size of radio equipment, therefore, reduction in the size of antennas is unavoidable.
As a construction for reducing the size of an antenna, for example, Japanese patent document JP-A-2003-152427 has been disclosed. The antenna disclosed in this patent document includes an internal conductor linearly extended and an external coiled conductor closely wound at a distance from the internal conductor with the internal conductor at the center of winding. The antenna is so constructed that it resonates at a specific frequency. Thus, the antenna is provided with a relatively high gain and further small and simple construction.
In case of this construction, the internal conductor is linearly extended, which limits antenna size reduction. The following case will be taken as an example. To reduce the size of radio equipment, the outer size of an antenna is reduced in the direction orthogonal to the direction of extension of the internal conductor. In this case, it is required to lengthen at least either of the internal conductor and the external coiled conductor to ensure an electrical length for resonance. Since the internal conductor is linear, however, the height of the antenna is significantly increased.
Meanwhile, the present applicants filed an application for Japanese patent regarding a following antenna structure under application No. JP-2005-188513 (Corresponding US publication No. US2006290590). In the disclosure of the application No. JP-2005-188513, the antenna is so constructed that using two elements, one as a signal wire and the other as a ground wire, an internal element is disposed inside a spirally extended external element at a distance between them. The internal element is in such a shape that it is spirally extended in the direction of the axis of the external element. By forming the internal element in a spiral shape as mentioned above, the band can be narrowed and the gain of the antenna can be enhanced. With substantially the same antenna gain, therefore, the antenna can be reduced in physical size more than antennas having a linear internal element can.
The above-mentioned antenna is so constructed that the following is implemented: an internal element is disposed inside a spirally extended external element with a predetermined distance between them; either of the two elements is used as a signal wire and the other is used as a ground wire. This type of antennas is antennas of so-called dipole structure, and the positional relationship between the two elements is important to the performance (resonance characteristic) of the dipole antennas. For example, when the distance is varied, the resonance frequency is varied and this has influence on the radiation characteristic. Further, a component in the direction perpendicular to the circuit board also has influence to the radiation characteristic. Therefore, when the inclination of an element to the circuit board is varied, the radiation characteristic is influenced.
However, in cases where two elements are separately mounted on a circuit board so that one end of each element is electrically connected with the wiring provided on the circuit board, problems arise. It takes much time and trouble to mount them, and further it is difficult to bring the two elements into desired positional relationship when they are mounted. Even if desired positional relationship can be obtained, it is difficult to hold the antenna in the desired positional relationship because of vibration produced in a use environment (e.g., in a vehicle-mounted environment).